In radio systems, diversity methods of different kinds are used for increasing the coverage area and/or capacity of the system. As to this publication, space diversity, i.e. antenna diversity, polarization diversity and multipath diversity are of interest. Space diversity signifies that antennas are positioned sufficiently far from each other to achieve a sufficient decorrelation between signals received via the separate antennas. An interesting kind of polarization diversity is implicit polarization, when a signal is sent on one polarization level, but received by cross-polarized antennas. Multipath diversity signifies a diversity created by multipath propagated signal components, this diversity being usable in a system, such as a CDMA system, in which the bandwidth of a signal is much wider than the coherent bandwidth of a channel.
In a CDMA system, a RAKE receiver is used for separating multipath propagated signal components at the reception. In general, the signal components must then be separated from each other at least by one part of a spreading code used, i.e. by a chip. The RAKE receiver comprises RAKE fingers and, in each of these fingers, despreading and diversity combination take place. In addition, the receiver comprises a delay estimator having a matched filter for each antenna branch and an allocation block for the RAKE fingers. In the matched filter, a signal, received by a spreading code used for signal spreading, is correlated by different delays, the timing of the spreading code then being changed for instance in steps of one chip. When the correlation is high, a multipath propagated signal component is found, which can then be received by the delay found.
On the radio path, the signal will include, besides the wanted signal, also noise and interference caused by other users or systems. In systems utilizing diversity, the influence of noise and interference can be decreased for instance by Maximal Ratio Combining (MRC) method, according to which method signals received via separate antennas are weighted in proportion to the signal power in the separate antenna branches. However, this method presupposes that the interference of each antenna is independent. This pre-supposition is not always true in practical cellular radio networks, but it is conceivable that the same interference is present at each antenna.
There is no such restriction on Interference Rejection Combining (IRC) method. However, the method has been used only in systems utilizing Time Division Multiple Access (TDMA) method, these systems often being incapable of separating multipath propagated signal components. IRC method signifies here an adaptive beam forming (optimal combination of signals), by which the signal power is maximized in proportion to the power of interference and noise, i.e. a Signal-to-Interference-and-Noise Ratio (SINR) is maximized.